For example, in an a digital camera set, various voltage levels of power supply voltages are needed in order to drive a motor, a memory, a speaker, a backlight, and the like. In order to meet the need, generally, there has been used a DC-DC converter that can generate various voltage levels of output voltages based on one input voltage.
Furthermore, in recent years, it has been required that low power consumption of a digital camera is achieved to enable it to operate for a long time. Therefore, it is being necessary to also reduce a consumption current at the time of low load of a DC-DC converter 1, which had not been taken into consideration before. That is, it is being necessary to enhance a power efficiency at the time of low load of the DC-DC converter 1.
A related art is disclosed in Patent Literature 1. FIG. 13 is a diagram showing an entire configuration of a switching power supply apparatus 200 disclosed in Patent Literature 1. An Nch driver 216 is a circuit that outputs a drive signal DRV_N to drive a synchronous rectification transistor MN201. The Nch driver 216 is further provided with a current direction detection unit that takes in a voltage V_LX1 of a node LX1, and detects a direction of a current flowing through an inductor L201 and the synchronous rectification transistor MN201.
An internal circuit of the Nch driver 216 is shown in FIG. 14. The Nch driver 216 has a current direction detection unit 2161 and a driver signal generation unit 2162. The current direction detection unit 2161 has: an I/V conversion circuit 2163 to which a fixed voltage VREG is supplied; a gate bias circuit 2164 that generates a bias voltage Vg1; an NMOS transistor MN202 that is provided between the node LX1 and the I/V conversion circuit 2163, and in which the bias voltage Vg1 has been applied to a gate thereof; and a comparator 2165 that compares a reference voltage Vref1 with a drain voltage V11 of the transistor MN202.
In the switching power supply apparatus 200, when a main transistor MP201 is turned on by a Pch driver 215 (the synchronous rectification transistor MN201 is turned off), a current flows in a capacitor C201 through the main transistor MP201 and the inductor L201 from a power supply device 211, and the capacitor C201 is charged. In addition, when the synchronous rectification transistor MN201 is turned on by the Nch driver 216 (the main transistor MP201 is turned off), by energy stored in the inductor L201, a current flows in the capacitor C201 through the synchronous rectification transistor MN201 and the inductor L201, and the capacitor C201 is charged. In a latter case, when detecting the change of the voltage V_LX1 of the node LX1, and determining that a direction of the current has reversed, the Nch driver 216 controls the synchronous rectification transistor MN201 to be turned off.
Operational waveforms of the Nch driver 216 are shown in FIG. 15. When a control signal PRDRV_N output from a power supply control circuit 214 of FIG. 13 changes as “L”→“H” (a control signal PRDRV_P changes as “L”→“H”, and the main transistor MP201 is turned off), the drive signal DRV_N changes as “L”→“H” by the driver signal generation unit 2162. As a result of it, the synchronous rectification transistor MN201 changes as off→on. As a result of it, a current flows toward the capacitor C201 through the inductor L201 from the synchronous rectification transistor MN201. Therefore, the voltage V_LX1 of the node LX1 becomes a negative voltage lower than a ground voltage GND.
After that, as time elapses, a current of the inductor L201 decreases, and a potential of the node LX1 also rises. When the current of the inductor L201 becomes zero, the voltage V_LX1 of the node LX1 also becomes zero, and after that, a reverse current begins to flow in a direction of the synchronous rectification transistor MN201 from the inductor L201.
Consequently, the reference voltage Vref1 is previously set to be the drain voltage V11 of the transistor MN202 at this time. An output of the comparator 2165 then inverts to be “H”→“L”, the drive signal DRV_N changes as “H”→“L” according thereto, and the synchronous rectification transistor MN201 is turned off. As described above, when the voltage V_LX1 of the node LX1 becomes zero, the reverse current that flows toward the synchronous rectification transistor MN201 from the inductor L201 is immediately blocked.
In addition to this, Patent Literature 2 discloses a switching power supply provided with a pair of synchronous rectification switching elements, which is characterized by being provided with a stop control unit for stopping one of the pair of synchronous rectification switching elements when an output voltage exceeds a predetermined voltage.
In addition, Patent Literature 3 discloses a synchronous rectification type switching regulator characterized by forcibly turning off a synchronous rectification transistor to be an interrupted state when detecting the occurrence of a current backflow.